Differential Quadrature Phase Shift Keying (DQPSK) transmission has emerged as an efficient modulation scheme for high-data rate optical transmission. DQPSK is a four-level version of Differential Phase Shift Keying (DPSK). DQPSK modulation transmits two bits for every symbol (bit combinations being 00, 01, 11 and 10). DQPSK has a narrower optical spectrum than DPSK, and tolerates more dispersion (both chromatic and polarization-mode), allows for stronger optical filtering, and enables closer channel spacing. For example, DQPSK allows processing of 40 Gbps data-rate in a 50 GHz channel spacing system. Additionally, a 112 Gbps signal can be transmitted using polarization multiplexing and DQPSK modulation at a 28 GBaud signal, correspondingly requiring electronic and optical components that need to support only a 28 Gbps bit rate. Advantageously, this allows for high-rate signal transmission exceeding the limitations of conventional direct binary modulation schemes.
Generally, a DQPSK modulator includes a combination of three modulator sections. These sections include an I-arm modulator for an in-phase data signal, a Q-arm modulator for a quadrature data signal, and a main modulator which is referred to as a phase modulator. The I-arm and Q-arm modulators are driven by independent data streams, and each of the modulators modulates different phases. For example, one modulator can perform modulation at 0 degrees and 180 degrees, and the other one can perform modulation at 90 degrees and 270 degrees (i.e., each modulator is always 180 degrees apart). DQPSK modulators can be built through discreet components (i.e., “stick built”) or as a single integrated unit.
The DQPSK modulator has three bias ports that need to be controlled for an optimal output including the I-arm and Q-arm modulators and the phase modulator. Bias is a DC voltage applied to the port. For example, the I-arm and Q-arm modulators are usually set for null point, and the phase modulator is set for quadrature point. Control is required of these three bias ports to compensate for environmental and/or systemic changes.
Conventional bias control mechanisms utilize dither tones which can be injected into arm modulators bias ports and/or modulator drivers. The state of this dither is subsequently monitored at the optical signal output of the modulator through an optical splitter with a tap feeding a photodiode. Disadvantageously, these conventional bias control mechanisms require multiple dither tones and multiple monitoring photodiodes. For example, to process these dither tone signals at the output of the modulator with the multiple monitoring photodiodes adds complexity related to fiber splicing, board space, and control. Each photodiode requires its own hardware and software to process dither signals.